Skip empty memory summaries in context window

[shuofengzhang]

What changed

• Skip emitting a summary system message when the summarizer returns an empty string.
• Add coverage to ensure empty summaries don't add extra context entries.

Why

• Avoids a noisy "Previous Summary:" placeholder when there's nothing to summarize.
• Keeps the context window focused on relevant messages.

Testing

• ✅ .................................................................. [100%]
  =============================== warnings summary ===============================
  .venv/lib/python3.14/site-packages/google/genai/types.py:43
  /home/shuofengzhangl/.openclaw/workspace/oss_pr_workflow/repos/study8677-antigravity-workspace-template/.venv/lib/python3.14/site-packages/google/genai/types.py:43: DeprecationWarning: '_UnionGenericAlias' is deprecated and slated for removal in Python 3.17
  VersionedUnionType = Union[builtin_types.UnionType, _UnionGenericAlias]

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
66 passed, 1 warning in 2.97s

[study8677]

@shuofengzhang Thanks for the great work on this PR — the change is clear, well-scoped, and the added test coverage is especially helpful. Really appreciate it.